The present application relates generally to micro-machined convective accelerometers, and more specifically to thermal accelerometers capable of detecting acceleration along multiple axes.
Thermal accelerometers are known that have the capability of detecting acceleration along multiple axes. For example, U.S. Pat. No. 6,182,509 (the '509 patent) discloses a thermal accelerometer device configured to detect acceleration in 2-axes. As disclosed in the '509 patent, the 2-axes thermal accelerometer comprises a substrate having a cavity etched therein, and a structure including a small heater plate and four temperature sensors suspended over the cavity. The heater plate is positioned at the center of the suspended structure, which is in a plane defined by X and Y axes. Further, two of the four temperature sensors are placed along the X axis on opposite sides of and at equal distances from the heater plate, while the other two temperature sensors are similarly placed along the Y axis on opposite sides of and at equal distances from the heater plate. In a typical mode of operation, electrical current is passed through the heater plate, which heats the surrounding air to generate a symmetrical temperature gradient in both the X and Y axes. Because the respective pairs of temperature sensors disposed on the X and Y axes are equidistant from the heater plate, the differential temperature between each pair of temperature sensors is initially zero. However, if an accelerating force is applied to the device in a direction parallel to the X-Y plane, then the temperature distribution of the air shifts. Specifically, when acceleration is applied in the X direction, a differential temperature is detected by the temperature sensors disposed on the X axis. Similarly, when acceleration is applied in the Y direction, a differential temperature is detected by the temperature sensors disposed on the Y axis. A bridge circuit and a differential amplifier are employed to generate signals representing the detected differential temperatures, which are proportional to the acceleration applied along the respective axes. According to the '509 patent, the 2-axes thermal accelerometer can be fabricated using known CMOS or bipolar processes, thereby allowing the accelerometer to be integrated with signal conditioning circuitry with relatively low cost.
One drawback of the 2-axes thermal accelerometer described in the '509 patent is that it cannot be easily configured to satisfy applications requiring acceleration sensing in three axes. For example, to sense acceleration in three dimensions, the '509 patent indicates that at least two thermal accelerometers may be oriented at right angles to each other, resulting in a 3-dimensional accelerometer device structure. Such 3-dimensional device structures are typically implemented using a motherboard and at least one daughter board. However, implementing a 3-axes thermal accelerometer with multiple printed circuit boards can significantly increase the cost and complexity of the device and reduce reliability, thereby making the thermal accelerometer unsuitable for use in many consumer electronics and automotive applications.
It would therefore be desirable to have a low cost thermal accelerometer that can be configured to provide up to three axes of acceleration sensing. Such a thermal accelerometer would provide high reliability, while avoiding the drawbacks of conventional thermal accelerometer devices.